Thalassa now suggests (if I understand you rightly) that this man Remington, wronged by Robert Turold many years before, was lurking outside in the darkness, and seized the opportunity of Thalassa's absence to enter the house and murder the man who had wronged him.

Have I got it right ?" "Yes," said Charles, "you have it right." "The story rests on Thalassa's bare statement, and Thalassa is a facile liar." Barrant's tone was scornful.
"He is not lying now," returned Charles, "and there is more than his bare statement to support his story.

Thalassa found his master cowering upstairs with fear in his study shortly before he met his death.

He then told Thalassa he had heard Remington's footsteps outside.

Thalassa laughed at him, but undoubtedly Remington was out there, waiting for his opportunity, which he took as soon as he saw Thalassa leave the house.